In electrical systems, impedance mismatches cause some of the transmitted energy to be reflected back to its source. The frequency of the signal determines the amount of energy that is reflected. Voltage Standing Wave Ratio (VSWR), a unitless ratio ranging from 1 to infinity, expresses the amount of reflected energy at the input or output of the device. A value of 1 indicates that all of the energy passes through.
VSWR is defined as the maximum voltage of the standing wave (Emax) to the minimum voltage of the standing wave (Emin). Any other value indicates that a portion of the energy is reflected. There is an equivalent definition for VSWR, which can also be expressed in terms of the Return Loss (RL). The return loss is the ratio of the power of the wave reflected to that of the incident wave. It is generally expressed in decibels (dB).
The Reflection Coefficient (ρ), a figure of merit related to VSWR and to the return loss, is defined as the ratio of the reflected voltage to the incident voltage. Finally, another measurement of mismatching of impedances is the Mismatch Loss (ML). This is a measure of how much the power of the transmitted wave is attenuated due to reflection. These four parameters are related through the following formulas